The prevailing methods for parameter adjustment of monolithic filters by thin-film deposition are based on deposition on both sides of the filter wafer. The present application addresses the parameter adjustment based on deposition on only one side of the filter. Prior art information on this type of approach includes the E. C. Thomson U.S. Pat. No. 4,343,827, which discloses a method for fine-tuning a monolithic crystal filter having a solid electrode on one side of the crystal wafer and a pair of split electrodes on the opposite side.
The method has several disadvantages:
1) It does not provide for bandwidth adjustment. While normally it is desirable to adjust both the center frequency and bandwidth of a filter, the referenced method can only adjust the center frequency. PA0 2) It requires deposition through at least two different-size mask apertures onto three fixed locations of the electrode, resulting in relatively complex deposition mechanisms. PA0 3) It requires close-tolerance alignment between electrode and apertures. This can become critical in high-frequency filters, where the electrode areas become very small and the tolerance requirements very exacting. PA0 4) It requires adherence to fixed areas of deposition. This restricts the freedom in optimizing the adjustment process. PA0 a) plating a thin film on substantially the full area of the resonator electrodes on said one side of the electrode pattern, for the purpose of adjusting the two resonator frequencies and the filter's center frequency, and PA0 b) plating a thin film of a width approximately equal to the inter-resonator gap in the center of said one side of the electrode pattern, for the purpose of adjusting the filter's bandwidth. PA0 a) plating a thin film onto a first area on said one side of said electrode pattern, PA0 b) measuring the effect of said plating on said electrical parameters, PA0 c) in response to said parameter measurements, changing said area of plating to another area of said one side of said electrode pattern, suitable for adjusting said parameters toward target values, PA0 d) plating a thin film onto said other area of said electrode pattern, PA0 e) repeating steps b, c, and d for further platings until the parameter targets are reached. PA0 a) a mask with a first and a second aperture, said first aperture having substantially the dimensions of one of said resonator electrodes, and said second aperture having substantially the dimensions of said inter-resonator gap, said mask facing said one side of the electrode pattern and having relative mobility with respect to said filter such as to allow the alignment of said first aperture opposite one and the other of said resonator electrodes, as well as the alignment of said second aperture opposite said inter-resonator gap, said respective alignments being called the plating positions, PA0 b) an evaporation source for plating a thin film through said apertures when they are aligned in their respective plating positions, PA0 c) means for electrically contacting the terminals and for measuring the resonator frequencies and bandwidth of said filter, PA0 d) means for evaluating said measurements and, in response to said evaluation, determining and causing the movements of said first and second apertures into respective plating positions for adjusting the center frequency and bandwidth of said filter. PA0 a) a mask with a single aperture, said aperture having a width preferably smaller than the width of the resonator electrodes and larger than the width of the inter-resonator gap, PA0 b) a mounting means for the filter and mask that affords mobility of said filter and mask relative to each other such as to allow alignment of said aperture opposite any part of said one side of the electrode pattern, said different alignments being called plating positions, PA0 c) an evaporation source for plating a thin film through said aperture when it is in a plating position, PA0 d) means for electrically contacting the terminals of said filter and for making measurements of the electrical parameters of said filter, PA0 e) a plating control means for PA0 e-1) evaluating said measurements when said aperture is in a plating position, and for determining a new plating position suitable for changing said electrical parameters to new values that converge toward target values, PA0 e-2) moving said aperture to said new plating position for controlled plating to said new parameter values, PA0 e-3) repeating steps e-1) and e-2) until the target values are reached. PA0 a) simpler construction, in that it requires a mask with only a single aperture. PA0 b) substantially reduced tolerance requirements for the initial mask alignment, in that the mask aperture may be substantially smaller than the filter electrode and can be moved in relation to it, PA0 c) substantially increased flexibility in the adjustment process, in that the deposition is not restricted within fixed boundaries but can be directed to any area of the filter electrode.
Items 2) and 3) will be explained in more detail in the "Description of the Invention". Items 1) and 4) are explained as follows:
The parameter adjustment process, generally refers to the adjustment of the motional inductances L1, L2, and L' of the filter's electrical equivalent circuit, which is shown in FIG. 1. The circuit represents two coupled resonators 1 and 2 and includes an input terminal 1, an output terminal 2, and a common-ground terminal 3. The inductances can be determined by various different measurement approaches, each of them comprising a set of 3 electrical measurements. The F. L. Sauerland U.S. Pat. No. 4,725,971 discloses an adjustment based on the measurement of 3 frequencies, which are related to the equivalent-circuit parameters as follows: ##EQU1## F1 and F2 are the frequencies of resonators 1 and 2, respectively. In the Thomson patent, they are called the open circuit resonance frequencies. For the following, we will assume that we are dealing with symmetric or approximately symmetric filters, which are characterized by EQU L1.apprxeq.L2.apprxeq.L; C1.apprxeq.C2.apprxeq.C; CO1.apprxeq.CO2.apprxeq.CO(3)
In this case, a "symmetric frequency" is defined as ##EQU2## a "center" or "midband" frequency is defined as EQU Fc=F1=F2,
and a "bandwidth" is defined as EQU Bandwidth=Fa-Fs, (5)
where Fa is called the "antisymmetric" frequency and defined as ##EQU3## Fa is dependent on and determined by the choice of F1, F2, and Fs. The frequencies F1, F2, Fa, and Fs are in this application also called "characteristic" frequencies.
In a symmetric filter, the adjustment normally aims at equalizing F1 and F2 to the filter's target center frequency and (Fa-Fs) to the filter's target bandwidth. For this, the inductances are increased by mass deposition on the filter electrodes.
FIG. 2 shows a cross section of a monolithic crystal filter MCF with two coupled resonators 1 and 2, comprising a pair of electrodes 5 and 7 and an inter-electrode gap on one side of the wafer, and a common-ground electrode 9 on the other side. This is the schematic that will be used in the subsequent text, although the common-ground electrode may be implemented in different ways, such as shown in FIGS. 3 and 4. Electrodes 5 and 7 and the electrode areas opposite to and congruent with electrodes 5 and 7 will also be called "resonator electrodes", and the complete electrode configuration will also be called "electrode pattern".
In the conventional approach, the inductance L1 is increased by plating substantially the full area of one or both electrodes covering resonator 1. According to equation (1), this decreases the frequency F1 of resonator 1. Further, L2 is increased by plating substantially the full area of one or both electrodes covering resonator 2. According to equation (2), this decreases the frequency F2 of resonator 2. L' can be increased by plating the area of the inter-resonator gap on either side of the wafer. According to equations (1), (2), and (4), this will decrease F1, F2, and Fs, and it will decrease Fs more than F1 and F2, while it will not affect Fa. As a result, the bandwidth will be increased according to equation (5).
Consider now the adjustment method according to the Thomson patent, "which comprises the steps of a) plating additional electrode material on a selected portion of the solid electrode to balance open circuit resonant frequencies of the filter, and b) plating additional electrode material on substantially the entire solid electrode to adjust the filter to a desired midband frequency."
There is no claim nor provision for bandwidth adjustment in this method, nor is there the possibility for a bandwidth adjustment independent of the resonator-frequency adjustment: in step b) the frequencies F1, F2, Fa, and Fs are lowered simultaneously according to equations (1) to (3), but they cannot be changed independently from each other. This means that in step b) the bandwidth change will be small, and either the resonator frequencies or the bandwidth--but not both--can be adjusted to target values.
As is well-known to people skilled in the art, the lack of bandwidth adjustment is a disadvantage in the adjustment of monolithic filters. Accordingly, one purpose of this invention is to eliminate this disadvantage and to provide a method for adjusting both center frequency and bandwidth of a monolithic filter by deposition on only one side of the filter blank.
A further discussion will explain the disadvantage mentioned in item 4) above. As equations (1), (2), and (4) show, there are coupling effects that link the change of one characteristic frequency to changes in other characteristic frequencies. Since in practice, the placement of the deposition cannot be controlled exactly, the equations cannot exactly express these coupling effects. However, the coupling effects can be measured and expressed in terms of a "coupling matrix". A sample matrix might be ##EQU4##
This matrix describes the effect of changes in F1, F2, and Fs on Fa, Fs, and the difference (F1-FS), which is to be adjusted to zero in a symmetric filter. Practical values for the coupling coefficient might be ##EQU5##
This matrix gives vital information for the adjustment process for a given set of circumstances. For instance, coefficient C23 is relatively small. This means that a change in Fs causes only a small changes in Fa. According to equation 6 this implies that increasing L' (in order to change Fs) produces only a small change in L1 and/or L2. This in turn implies that in the case described by matrix (8), L' is being increased by deposition of a narrow strip in the vicinity of the inter-resonator gap, i.e. without simultaneously increasing L1 and/or L2. In the Thomson method, the C23 value would be close to 1, since the deposition covers the whole solid electrode, thereby decreasing L1 and L2 as well as L'. According to equations (1) to (6), this produces a major change in F1 and F2 and only a minor change in the bandwidth.
There are conventional adjustment methods, based on deposition on both sides of the filter, that provide for adjustment of both center frequency and bandwidth. They are normally based on deposition onto 3 fixed areas of the filter electrodes, and they are normally done in steps, alternating the plating between the 3 fixed electrode areas. A typical approach might first alternate between plating resonators 1 and 2 to equalize F1 and F2 and adjust them to an intermediate target, then adjust Fs, then if necessary repeat the steps until F1, F2, and Fs reach their final targets. During these steps, the coupling coefficients are essentially fixed within relatively narrow boundaries because the deposition areas are fixed. This is a limitation of the conventional approach. If the deposition areas were variable, the coupling coefficients could be changed and optimized during the process, and the number of plating steps and the total plating could be reduced. This is further explained in the description of the invention.
While there are various other conventional methods for adjusting monolithic filters, they all appear to share at least some or all of the described disadvantages. Accordingly, the primary purpose of this invention is to provide new and improved methods and apparatus for adjusting the electrical parameters of monolithic filters that are free of the described disadvantages.